This invention relates generally to detection systems for detecting the presence of an object in a monitored zone and, more particularly, to an infrared detection system using infrared signals at multiple frequencies to discriminate between light reflected from an object within the monitored zone and other light and having means to selectively vary the boundaries of the monitored zone.
In many known photoelectric synchronous detection systems, a pulsed optical beam signal is transmitted into a volume or zone of space being monitored, typically by using an LED which is activated by a square wave or low duty factor pulse generator/oscillator. An optical photodetector is aimed into the monitored zone with a field of view which includes the pulsed LED beam so that it will receive any reflection of that signal to detect the presence of an object in the monitored zone. Such a system uses triangulation to discriminate between light reflected from objects within the monitored zone and light emanating from beyond the boundaries of the monitored zone, and is shown in U.S. Pat. No. 5,463,384xe2x80x94Juds.
To screen out noise and signals from sources other than a reflection from an object (e.g. other electrical or optical sources), synchronous receivers are used which operate the receiver only when a reflection of the pulsed signal is expected. This blocks any response resulting from detection of light energy from other sources during intervals when no reflected pulsed signal is possible.
To reject possible detection of intrinsic random circuit noise and detector shot noise, a fixed detection threshold is imposed on the system at a level above the expected intrinsic random noise levels seen by the detection circuit. This allows the detection circuit to ignore this noise. The probability of false detection due to noise is a function of the threshold level relative to the actual noise level, the amplitude of which is generally a Gaussian distribution.
Other examples of fixed threshold photoelectric detection systems are found in U.S. Pat. No. 4,356,393xe2x80x94Fayfield, U.S. Pat. No. 4,851,660xe2x80x94Juds, U.S. Pat. No. 4,851,661xe2x80x94Everett, Jr., U.S. Pat. No. 4,990,895xe2x80x94Juds, and U.S. Pat. No. 5,122,796xe2x80x94Beggs et al. Although these fixed threshold synchronous detection systems have been found useful for most photoelectric sensor applications, they are not sufficiently accurate in a situation where high receiver sensitivity is desired in an operating environment where the noise level is highly inconsistent and randomly variable.
In such an environment, detector system performance is handicapped by the necessity of tailoring detection threshold levels to performing in an environment of the worst expected noise conditions to assure a satisfactory level of noise rejection. This situation exists when the detection system is used for vehicle detection in an outdoors operating environment. Such a system used to detect vehicles in a driver""s blind spot will encounter a wide variation in noise resulting from ambient light conditions that range from pitch dark nighttime, to 8500 ft-cdls of sunlight reflected from a white surface, to as high as 70,000 ft-cdls of sunlight reflecting from a wet road surface. Also, such systems can be fooled by the presence of atmospheric backscatter caused, for example, by heavy fog or snow, to falsely indicate the presence of a vehicle in the blind spot. Since false detects by such systems renders them unreliable to a vehicle driver, elimination of false detects is an important goal.
In a blind spot detection system, the reflectivity of detected target vehicles will vary wildly, as will ambient lighting conditions. Thus, such a system will be required to detect vehicles that range in reflectivity from black to white, in lighting conditions that vary from pitch-dark nighttime to bright sunlight. Thus detection requirements range from a black vehicle at nighttime to a white vehicle in bright sunlight.
In the dark of night very little DC photocurrent is produced in the detectors, resulting in very little shot noise. However, operation in bright daylight will result in quite significant DC current in the receiver photodiodes, resulting in high shot noise levels. When the receiver views a white target vehicle in bright sunlight, the photocurrent generates shot noise which is many times greater than the intrinsic electronic noise of the receiver amplifier itself. To avoid false detection caused by a high level of shot noise, the required threshold must be quite large in comparison the worst case shot noise. This high threshold results in low system capability of detecting very dark, low reflective targets in all lighting conditions.
There have been several attempts to overcome the operational problems caused by this wide variation in system noise levels. These involve providing the detection system with some form of adaptive adjustment based on a measurement of the noise amplitude characteristics which are then used to set the detection threshold of the receiver. The resulting adaptive threshold receiver optimizes its sensitivity relative to the ambient measured receiver noise to maintain signal reception integrity. Examples of such systems are found in U.S. Pat. No. 3,999,083xe2x80x94Bumgardner, U.S. Pat. No. 4,142,116xe2x80x94Hardy et al, U.S. Pat. No. 4,992,675xe2x80x94Conner et al, and U.S. Pat. No. 5,337,251xe2x80x94Pastor.
Such systems are quite expensive, since they require the addition of circuitry to continually measure noise, to block such measurement and maintain the prior measurement when an actual signal is detected, and to feed measured levels back to the variable gain stage. This circuitry adds components and assembly labor, and increases system size.
Vehicle blind spot detector systems such as disclosed in the above-mentioned patents utilize both driver-side and passenger-side detectors. One system comprises sets of six emitter-detector pairs in a module, the detectors being, pairs of photodiodes of opposite polarity. The effective range of the system is determined by the geometry of these components. These components are quite small and require holding very precise tolerances during manufacturing to maintain their geometry.
It has also been proposed to provide a blind spot detector featuring a synchronous pulse detection system having an adaptive threshold that is inherently controlled by the statistical nature of the receiver noise to optimize sensitivity of the system receiver and maintain an acceptably low false detection rate. A multi-test zero threshold detector checks the combined noise and pulse response of a bandwidth-limited receiver at two or more spaced time points which are timed by pulse emission to correspond with expected maximum and minimum voltage peak and flyback responses from reflections of the emitted pulses. An up/down counter is employed to count up only if the comparator reports the correct polarity of the responses, and counts down for all other responses. The up/down counter is heavily biased to count down until the received signal is large enough relative to the noise to overcome the negative count bias and count up to produce a detect signal. In this system, the false detection rate in the absence of a valid signal decreases exponentially with the length of the counter. Such a system is disclosed in PCT/US97/20637, the disclosure of which is incorporated herein by reference.
This detector system also operates on the geometric arrangement of the emitters and photosensors. Since triangulation is used to discriminate between sensed reflections from objects within the zone and from beyond the monitored zone, precise placement of these elements is critical. Also, since three lenses are required, the unit remains bulky and must be mounted on or within the vehicle body, usually at the taillights.
Systems using triangulation require a second receiver for each emitter to be sufficiently insensitive to reflections from non-uniform objects in the monitored zone. Such double triangulation systems not only bear an added cost burden for the extra circuitry and components, but also increases the unit""s physical size, which makes it unattractive to space-conscious automobile manufacturers.
There is a need for a detector system which is small and compact enough to be placed in or on the outside rearview mirrors of a vehicle. There is also a need for a detector system that does not require precise relative placement of the emitters and photodetectors, nor the use of double triangulation, thus enabling the use of fewer and less expensive components, smaller unit size, and minimized manufacturing cost. There is also a need for a detector system that incorporates built-in adjustments for selectively varying the boundaries of the monitored zone.
These prior art light energy detector systems are but one segment of photo-optical ranging technology. In another segment, a short pulse laser beam is emitted toward a distant object and the time delay of the reflection of that beam is determined. From this information, the distance of the object can be determined, such as shown in U.S. Pat. Nos. 2,234,329; 3,723,002; 4,634,272; 5,179,286; 5,699,151 and 5,724,141. Other systems measure the phase delay of reflected fixed frequency modulated light, as in U.S. Pat. Nos. 3,778,159; 3,888,588; 4,146,328; 5,194,906 and 5,239,353.
Further systems measure the frequency of an oscillator and include the optical path in the feedback loop, as illustrated in U.S. Pat. Nos. 3,649,123; 3,739,628; 3,778,160; 5,125,736 and 5,309,212. Yet other systems measure the difference frequency produced by mixing the transmission frequency with the return frequency, which is known as FM-CW or chirp modulation, as shown in U.S. Pat. Nos. 3,647,298 and 4,721,385.
These range sensing systems were mainly developed for surveying and military applications, which require precise determinations of long distances. Although such systems could find applicability for blind spot detection applications, they require equipment that is both too bulky and too expensive for commercial practicability.
However, similar systems have been proposed for automotive use. One such system has been proposed for use in vehicles to detect and track a frontal object. As shown in U.S. Pat. No. 5,461,357, a computer tracks the relative speeds of the vehicle and a detected object to judge if the object presents a hazard to the vehicle. Another system, shown in U.S. Pat. No. 5,260,682, uses the phase shift principle to determine the distance between a vehicle and an approaching object. The rate of change of this distance is used, along with the vehicle speed to determine the speed of the object. Both of these systems are exceedingly complex and prohibitively expensive. As a result, they have found no commercial applications to date.
An additional problem that such distance measurement systems must overcome is atmospheric backscatter. In an automotive setting, this backscatter takes the form of fog, snow, and road spray or mist. This problem is exacerbated by the conflicting objectives of (a) sensing a very low reflectivity car at the far boundary of the monitored zone and (b) maintaining immunity to false detections in the presence of heavy fog, mist or snow. Although reflectivity if of atmospheric backscatter is usually small when compared to a black car at the far boundary of the monitored zone, the returned signal in a photoelectric system decreases with the square of the range in the far field (beyond a few feet). Thus, the photoelectric response from an object at 3 feet is 49 times stronger than the response from an object at 21 feet. This characteristic aggravates the problem, since sensitivity to atmospheric backscatter at close range is much better than at the far boundary of the monitored zone and makes balancing these objectives virtually impossible without some form of compensation.
This problem is addressed in U.S. Pat. Nos. 5,724,141; 5,311,012; 5,354,983 and 5,418,359. However, the solutions proposed require additional components and circuitry which render them so costly as to be impracticable for automotive use.
It is an object of this invention to provide a photoelectric object detector that does not require precise relative placement of the emitters and photodetectors, thus enabling less expensive components and manufacturing.
It is another object of this invention to provide a photoelectric object detector having means for selectively varying the boundaries of the monitored zone.
It is yet another object of this invention to provide a photoelectric detector which is small and compact enough to be placed in the outside rearview mirrors of a vehicle.
It is a further object of this invention to use the phase shift principle to determine the presence of an object within the boundaries of a monitored zone.
It is a still further object of this invention to provide a detection system in which a detection signal caused by atmospheric backscatter is effectively nulled.
This invention provides a system which eliminates the need to measure the specific range of a detected object and only determines if the object is in the monitored zone. The system uses a simplified phase delay measurement architecture in which the frequencies used are chosen so that a returned signal reflected by an object in the monitored zone produces a positive demodulation signal for each of the frequencies used, eliminating the complexities of prior art systems.
It also provides a system which neutralizes the effect of atmospheric backscatter by strategically positioning the emitter-receiver field of view overlap and choosing natural detection nulls tailored to monitored zone characteristics.
In one aspect, this invention features an electro-optical detection system for detecting objects within the boundaries of a monitored zone comprising an emitter and photodetector pair, wherein the emitter emits a beam of pulses of light energy at multiple frequencies into the monitored zone, and the photodetector detects light energy including light energy from the beam that is reflected from an object within the monitored zone and generates light detection signals, and a controller for operating the emitter and photodetector pair and for generating a phase-delayed reference signal at each frequency. The controller mixes the light detection signals and the reference signals to produce a phase difference signal for each frequency that varies in polarity sinusoidally with the distance to the object, and compares the polarity of these phase difference signals to determine the presence of an object in the monitored zone.
Preferably, the detection system includes a plurality of emitter and detector pairs, each monitoring a unique portion of the monitored zone, and the controller is operable to adjust the phase of the reference signals for each to vary the effective boundaries of the monitored zone.
In yet another aspect of this invention, the controller includes a phase shifter for phase shifting the reference signal to equal the expected phase shift of an emitted signal reflected from an object at a boundary of the monitored zone, enabling the comparator to determine whether received light energy is reflected from an object within or outside said boundary.
In still another aspect of this invention, the controller includes a programmable phase shifter which generates sequential reference signals at each frequency that are phase shifted to equal the expected phase shift of corresponding light energy pulses reflected from an object and at the far boundary of the zone, thus enabling the comparator to determine whether the phase shift of received light energy is greater or lesser than the reference signal.
Preferably, the detection system is mounted on each of the vehicle""s outside rearview mirrors.
In a further aspect of this invention, collision avoidance apparatus is mounted on a host vehicle to detect the presence of an adjacent vehicle within a monitored zone and has a sensor module which includes at least one discrete sensor system for monitoring a unique portion of the monitored zone. An emitter emits a fixed frequency modulated beam of light energy with a predetermined burst length into the unique portion of the monitored zone, and a receiver which has a field of view substantially aligned with the emitted beam, senses light energy and generates a received signal. A frequency generator produces at least one fixed frequency, a phase shifter produces a phase shifted reference signal with a predetermined phase delay relationship to said fixed frequency, and a mixer produces demodulation signals from each received signal and each phase shifted reference signal. The radiation pattern of the emitted beam and the field of view of the receiver have a fixed optical overlap area, and the fixed frequency and the predetermined phase delay are chosen to substantially produce a null received signal resulting from reflections of the emitted beam from uniformly distributed atmospheric backscatter within the optical overlap area.
These and further objects and features of this invention will become more readily apparent upon reference to the following detailed description of a preferred embodiment, as illustrated in the accompanying drawings, in which: